The invention relates generally to electronic circuits and in particular to an electronic circuit configured as a current mirror and more particularly to a current mirror adapted for implementation where there is low headroom.
A current mirror is a current controlled circuit which when fed with an input current outputs and an identical current. As such, they have many applications and are widely used in Integrated Circuit (IC) design.
FIG. 1 shows one such known current mirror 100 comprising two matching bipolar npn transistors Q1, Q2, whose bases are linked. When an input current (Iin) is fed into the diode connected first transistor Q1, it forces Q1 to generate a forward base emitter voltage which is a function of the applied current and which is applied directly to the base-emitter junction of the second transistor Q2, causing it to sink an almost identical collector current (Isink). It will be understood that Q2 thus acts as a current sink that is controlled by Iin, but has the advantage of acting as such even at low collector voltages. A problem with this current mirror is the high variability in the output current with changes in the output voltage (namely the collector emitter voltage).
FIG. 2 shows an alternative current mirror utilizing two Field Effect Transistor (FET) devices, Mn1, MN2. Again, this circuit tries to satisfy the following equation:
Iout=kIinxe2x80x83xe2x80x83(equation 1) 
where Iin is the current of the current source and lout is the output current flowing from the voltage source. This is difficult to achieve using circuitry such as that of FIG. 2 in that variations in drain-source voltages can cause a mismatch between input and output transistors resulting in high variability in the output current with changes in the output voltage. A modification to this circuit is shown in FIG. 3, where a second current mirror comprising a second set of transistors MN3, MN 4 is included. These additional devices ensure that the voltages measured across the drain and source (Vsd) of both MN1 and MN2 are equivalent. In doing so, the current mirror""s output current becomes less sensitive to changes in its output voltage. Unfortunately this implementation, although solving the problems associated with the circuits of FIGS. 1 and 2 and solving equation 1, requires a lot of headroom at the output of the circuitry so as to cater for the third transistor MN3 at the output; resulting in a stacking of two transistors thereby requiring two Vsd voltages.
A second problem with known current mirrors is that most of the known implementations require the devices with which they are made to remain in a region of operation where they have reasonably large output impedance. A simple MOS mirror, for example, will have very inaccurate current gain and poor output impedance if the output device leaves saturation. Unfortunately, in most mirrors, gain accuracy, statistical matching and output impedance all degrade as the headroom over them is decreased. While this is true generally, it is especially true if the devices leave the xe2x80x9cnormalxe2x80x9d area of operation.
There is therefore a requirement for a device that overcomes the problems associated with known circuits by providing a current mirror which may be used in situations where low headroom is available.
In accordance with one embodiment of the present invention a current mirror is provided with an input current and an output current having a defined relationship with the input current and which is adapted to be operable in situations where low headroom is available. The mirror comprises a first and second device, each device having a primary control, a primary output and a secondary output. The primary control of each device is connected at the same potential and the secondary output of the two devices is connected at the same potential. The primary output of the second device forms the output of the mirror. A control element having a first and second input and a first and second output is also provided, the first input being connected to the input current of the mirror, the second input being connected to the primary output of the second device, the first output controlling the potential at the primary control of the first and second devices, and the second output being connected to the primary output of the first device. The outputs of the control element are adapted to force the primary output current of the first device to match a defined ratio of the input current and the voltage on the primary output of the first device to match the voltage at the primary output of the second device, thereby maintaining the defined relationship between the input and output of the mirror.
In a first embodiment the first and second devices are Field Effect Transistors (FETs); the primary control of each FET being the Gate, the primary outputxe2x80x94the Drain, and the secondary outputxe2x80x94the Source.
In a second embodiment of the present invention the first and second devices are bipolar devices; the primary control of each bipolar transistor being the base, the primary outputxe2x80x94the collector and the secondary output the emitter.
Desirably, the second control block input is a high impedance input thereby minimizing the current difference between the output current of the second device and the output of the current mirror.
The control element typically comprises an amplifier and a FET transistor, the amplifier having a first input connected to the drain of the second transistor and a second input connected to the drain of the first transistor, the amplifier having an output connected to the gate of a third transistor, the source of the third transistor being connected to the drain of the first transistor, the drain of the third transistor being connected to the input current and additionally being connected to the common gate terminals of the first and second transistors, the amplifier output changing the gate potential on detection of changes to the input of the amplifier so as to maintain a defined ratio between the drain current of the first transistor and the input current and the voltage on the drain of the first transistor with that of the voltage on the drain of the second transistor thereby maintaining a defined relationship between the input and output of the mirror.
The current mirror of the present invention is advantageous over prior art implementations in that the control circuitry changes the primary control of the first and second transistors to compensate for changes in the output voltage. This results in a higher effective output impedance for the mirror without adding additional devices to the output leg of the mirror.
As a single transistor is utilized at the output it is possible to implement the mirror of the present invention in devices having low output headroom, it is also possible to operate the output transistor in a region other than that which would be normal for most mirrors without a significant degradation in performance as the control element compensates for variations in region of operation in the output device. When implemented using FET devices it will be appreciated that the device may be operated over broader ranges as the control element effects a broadening of the useful region of operation of the transistor beyond what would normally be considered a normal, or saturated, region of operation.
These and other features of the present invention will now be described with reference to the following Figures which are illustrative of the present invention but not intended to limit the present invention to that described.